1. Field of the Invention
The present invention is related to switch mode power supplies and more particularly to small systems power supplies for entry control systems and the like.
2. Background Description
Apartment buildings, office buildings, condominium complexes, gated residential communities, industrial parks and other secured locations often include an entrance access control system. One type of access control system, known as a telephone entry system (TES), provides building security as well as tenant access control to a particular building, apartment complex, etc. The access control system controls entry at one or more other building entry points, e.g., doors, garage doors, etc. A typical access control system includes a main control unit located at a primary entrance and depending on the size of the structure or area being monitored additional remote units may be provided to control remotely located doors. The access control system may also monitor the connected entry points for unauthorized access. For a TES type access control system, visitors wishing to enter the building/complex contact tenants or other building personnel over the TES who are capable of admitting the visitor by remotely unlocking the entrance, e.g., from the tenant""s apartment.
The main control unit controls the main entrance and may include a keypad and auto-dialer and be connected to a public telephone line. Remote units, typically communicate with the main unit to provide remote access to authorized personnel. The main unit can identify tenants seeking entry by personal access code, authorize entry, monitor for unauthorized entry at the remote doors, etc. A tenant directory may be displayed on the control unit itself or on an adjacent sign. The directory includes tenant codes which are corresponding directory numbers for each person, business or other parties in the building (tenants) that are capable of unlocking the entrance.
When a visitor enters a tenant code into the keypad, the main control unit automatically dials the corresponding tenant""s telephone number. Then, the called tenant has an opportunity to establish the identity of the visitor. The tenant, using the same everyday telephone upon which the call was received, unlocks the entrance, e.g., by pressing a predetermined telephone number on the keypad. Currently, each main unit and connected remote units includes its own low voltage power supply.
FIG. 1A is a schematic of a state of the art power supply 50. Alternating Current (AC) is supplied through a typical, small wall transformer stepping voltage down from a 110 VAC to 9 VAC in this example. The 9 VAC is provided to a full wave rectifier 52, a bridge rectifier in this example. The bridge rectifier 52 rectifies the 9 VAC to provide a 9V rectified DC, which is passed to a capacitor filter 54. The capacitor 54 filters transients from the rectified voltage, providing an unregulated 9 volts DC that may include an acceptable ripple voltage. In this example, a voltage regulator powered by the unregulated 9V includes a zener diode 56 biasing a transistor 58 to provide 3V. The 3 volt supply regulator, converts the unregulated 9 volts to a clean 3 volt regulated output.
Once the capacitor 54 charges, essentially to the peak magnitude of the input AC, i.e., the upper magnitude of the rectified DC, the unregulated voltage across the capacitor 54 remains essentially constant. The unregulated voltage appears as a relatively clean DC (i.e., constant) voltage as long as the DC is unloaded. As long as the capacitor voltage remains constant, i.e., at no load, no current passes from the AC input through the rectifier to the unregulated voltage. However, the unregulated DC powers a voltage regulator which converts the unregulated DC to a supply voltage, e.g., 3 volts in this example. Typically, the regulator draws current for a load. Load current discharges the capacitor 54 slightly between AC peaks, causing the ripple voltage on the regulated DC. So, the ripple voltage is symptomatic of load current discharging the capacitor 54 between AC voltage peaks and, then, recharging the capacitor 54 during each peak.
The amount of ripple is set by design. The load current may be specified small enough that ripple voltage on the unregulated DC voltage is less than 5-10%. Output current flows constantly from the capacitor 54; and, input current flows through the rectifier 52 only during a small portion of each AC cycle, i.e., at voltage peaks. Unfortunately, during these short periods when rectifier current does flow, very high input current flows.
FIG. 1B is a comparison of the AC input voltage 60, unfiltered DC 62, the filtered unregulated supply voltage 64 and current 66 through the rectifier generating the unregulated voltage and regulated voltages. For example, for a 3V DC supply to provide 0.1 Amps (0.3W), regulator transistor 58 continuously draws 0.1 Amps from the 9V unregulated supply (0.9W). For a ripple voltage less than 10% (0.9V), the total charge removed by load current (between peaks) must be replaced, roughly during 13% of each half cycle at each peak. Therefore, remembering that I=Cdv/dt, the current necessary to recharge the capacitor 54, averages about 0.8 Amps during that 13% recharge period of each cycle. So, much higher peak current flows through the rectifier 52 during the peak periods than the DC current being supplied. Further, since this peak current both starts (when the input voltage rises above the unregulated supply voltage) and terminates even more rapidly (when the input line voltage begins to reverse polarity) reactance from input path inductances can become significant.
Thus, to avoid component failure from high current or reduced supply voltages from inductance and resistive line losses (i.e., voltage drops across input line impedance), the bridge rectifier must be capable of handling brief but high peak currents and a higher gage wire must be used than the actual consumed power otherwise would merit. Also, if the AC input is a low voltage, e.g., 9 volts from a plug-in wall transformer, instead of typical house current of 110 volts, the input transformer must also be capable of handling this large peak current. Since input components such as the transformer must be heavier duty and the wire between the input transformer and the bridge rectifier must be a higher gage, they are more expensive than the average current (and correspondingly the power consumed) would otherwise necessitate.
Furthermore, power companies discourage high peak current requirements. The current from a single such small wall transformer to power a DC supply may account for an insignificant percentage of the total current required for a single house and so may not be a concern. However, aggregated over an entire neighborhood, where a large number of these small transformers are powering small DC power supplies, each adding higher peak currents, supplying the aggregate current can become a major problem. This is becoming the norm as small low voltage appliances are becoming popular. So, just as larger components are required for providing AC to these DC supplies, power companies must use larger generators, power lines, etc., to satisfy the aggregate peak currents for these neighborhoods.
The power consumption concern with these prior art supplies is quantified as power factor. Power factor (PF) reflects how efficiently electricity is being used. The power provided (apparent power) in kilovolt amperes (kVA) includes both the actual power used (consumed) in kilowatts (kW) as well as reactive power (also in kVA) and PF=consumed/apparent. Typically, power factor is lower in the presence of non-linear devices such as solid state or switch mode power supplies.
Power companies charge customers based on the apparent power supplied to a particular facility or home not on the actual power consumed. It is important to consumers to keep power costs low and so, to efficiently consume the power being provided. In other words, ideally, consumed power equals the apparent power and the power factor is one (1) or as close to 1 as possible. Unfortunately, the power factor for these low voltage DC supplies is much less than 1 and is roughly the DC power being consumed, which is proportional to the average current supplied, divided by the magnitude power supplied, which is proportional to the peak line current. Thus, as noted above, the power typical factor may be 10-15% or very much less.
Another problem frequently encountered when upgrading or changing access control systems is that the existing power source may be incompatible with the new access system, e.g. AC versus DC, 12 volts versus 9 volts, 50 Hz versus 60 Hz, etc. It may be difficult to locate the original power source or the low voltage power transformer supplying the power source. Locating and/or replacing the existing power sources may require extensive time to search for the power transformer and, once it is located, removing and replacing it may require extensive digging and upheaval of the surrounding area.
Thus, there is a need for an access control system which can operate on any power that may be available without requiring locating and replacing the original power transformer. There is also a need for power supplies with a power factor approaching one.
Accordingly, it is a purpose of the present invention to simplify TES installation;
It is another purpose of the invention to improve power factor for switch mode power supplies;
It is yet another purpose of the invention to improve TES power consumption;
It is yet another purpose of the invention to reduce power wasted in a TES;
It is yet another purpose of the invention to increase TES power supply efficiency.
The present invention is a switch mode power supply that may be used for a telephone entry system (TES) or the like. Rectified DC from a full wave rectifier is passed to a pair of inductors that are alternately sinking and sourcing current thereby drawing a more uniform input current because current constantly flows through the full wave rectifier to both of the inductors. A pair of FETs, each connected to one of the pair of inductors, are alternately turned on and off such that current from the current sinking inductor is shunted to ground. When one inductor is sinking current (i.e., through an active one the FETs) the other inductor is sourcing current to charge a filter capacitor and thereby provide an essentially constant unregulated voltage. The unregulated voltage at the capacitor is supplied to voltage regulators which in turn supply regulated voltage. At steady state, all current being supplied by the sourcing inductor is passed directly to and through the voltage regulators.
Thus, the power supply of the present invention has a power factor that is nearer to 1.0. Advantageously, input transformers and connecting wires may be smaller, rated for lower current, lower gage and so less expensive. Further, a prior art low voltage supply may be replaced with a preferred embodiment switching mode power supply using existing transformers and wiring, thus avoiding the expense and problems of locating, replacing and rewiring the existing transformer.